Thermal type air flow meter having a reinforcing structure provided on the base member between the board fixing part and the secondary passage constituting part

ABSTRACT

A thermal type air flow meter, capable of suppressing deformation of a base member at the time of molding to secure dimensional accuracy and reduce an influence of a dimension change on measuring accuracy, is to be placed in an intake passage of an internal combustion engine. The thermal type of air flow meter includes a resin component having a secondary passage into which part of the air passing through the intake passage can flow, and a circuit board configured to output a signal according to an air flow rate based on a signal from a flow rate detecting element disposed on the circuit board. The circuit board on which the flow rate detecting element is disposed is installed so that the flow rate detecting element is placed in the secondary passage. The resin component includes a board fixing part to which the circuit board is fixed, a secondary passage constituting part that is a part of the secondary passage, and ribs provided on a rear surface, which is a side opposite to a surface of the board fixing part, with the ribs forming a grid pattern.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/470,034, filed Aug. 27, 2014, the entire disclosure of which isincorporated herein by reference, the priority of which is claimed,which is a continuation of U.S. application Ser. No. 13/228,039, filedSep. 8, 2011, the priority of which is claimed, which claims priorityfrom Japanese Patent Application No. 2010-201525, filed Sep. 9, 2010,the priority of which is also claimed here.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a thermal type air flow meter thatmeasures the flow rate of air, for example, to a thermal type air flowmeter that is attached to an intake pipe of an internal combustionengine and measures the flow rate of intake air supplied to an engine.

Background Art

A thermal type air flow meter that measures the flow rate of intake airsupplied to an internal combustion engine is attached for use to part ofan intake system of an internal combustion engine. The thermal type airflow meter has a mechanism of, for example, causing a flow ratedetecting element such as a heat-generating resistor to generate heatand measuring the flow rate of passing air on the basis of the amount ofheat radiation therefrom to the air. Accordingly, it is necessary toconsider protection of the flow rate detecting element fromcontaminating substances and other factors during long-term use andsecurement of the flow rate measuring accuracy. Further, it is alsonecessary to consider intake air pulsations such as a backward flow thatare generated in an intake pipe of the internal combustion engine whenthe intake air is pulsated by opening/closing of an intake/exhaust valveof an engine and the intake pipe resonates with the rotation frequencyof the engine.

In a conventional thermal type air flow meter included in an internalcombustion engine, JP Patent No. 3523022 describes that a flow ratedetecting element, which is placed in a secondary air passage, isprotected from contaminating substances, a backward flow, and otherfactors. Further, in recent years, from the aspect of purification ofexhaust gas and improvement of fuel efficiency, highly accuratemeasurement of an intake air flow rate is required, so that a thermaltype air flow meter that accurately measures even a backward flowgenerated in an intake pipe is necessary.

In addition, with regard to a thermal type air flow meter having such arib structure as described in the present invention, JP PatentPublication (Kokai) No. 05-302839 A (1993) proposes a structure in whicha rib is provided on an element rear surface in order to reinforceresistance to vibrations of a cantilever-like element. Further, JPPatent Publication (Kokai) No. 2002-107201 A can be exemplified as acountermeasure against deformation due to a sink occurring at the timeof injection molding. According to this countermeasure, such a sinkoccurring at the time of injection molding is reduced by providing arib, and the degree of flatness of an opposed surface is accordinglyimproved, whereby air is prevented from leaking from an insertion partof a flow meter into an intake system.

A thermal type air flow meter measures a backward flow in a state wherea detecting element is placed inside of a secondary air passage, andhence the secondary air passage of the thermal type air flow meter isrequired to have a complicated structure including a bent passage partand a narrowed-down shape. Meanwhile, a reduction in cost of the thermaltype air flow meter is demanded from the market at the same time. Thatis, it is necessary to overcome a contradictory problem that thecomplicated structure is required while the reduction in cost is aimedat at the same time.

In order to achieve the complicated structure of the secondary airpassage, a conceivable idea involves increasing the number of componentsand combining the components to thereby constitute the secondary airpassage. The increase in the number of components, however, leads to anincrease in cost, and in order to achieve the reduction in cost, it isnecessary to achieve the complicated structure without increasing thenumber of components. This enables not only a reduction in cost of thecomponents but also a reduction in assembly man-hours, so that it ispossible to reduce cost of the thermal type air flow meter or suppressan increase in cost thereof.

Taking the structure described in JP Patent No. 3523022 as an example,the thermal type air flow meter includes, as main components, sixcomponents of 1) the flow rate detecting element, 2) a housing, 3) acircuit board, 4) a metal base, 5) a secondary air passage member, and6) a cover.

Among the six components, components mainly constituting the secondaryair passage are two components of a portion of the metal base and thesecondary air passage member. The secondary air passage member is formedby resin molding, and hence a complicated passage structure is easy toachieve.

In contrast, the metal base is a component having the portion thatconstitutes the secondary air passage and also having another portionthat has a function of bonding and fixing the circuit board, thehousing, and other components. In addition, the metal base is formed bypressing a flat plate-like metal material, and hence the metal base isadvantageous to maintain a flat surface for the bonding and the like butis disadvantageous to achieve the complicated structure of the secondaryair passage.

Accordingly, it is conceivable to form by resin molding only a portionof the secondary air passage constituted by the metal base membersimilarly to the secondary air passage member, but in order to place theflow rate detecting element inside of the secondary air passage, thisportion needs to be newly prepared as a separate component, resulting inthe increase in the number of components and the increase in assemblyman-hours, and the increase in cost of the thermal type air flow metercannot be avoided.

In order to solve these problems, it is conceivable to form the metalbase as a resin base by resin molding, but the resin easily becomesthicker in the portion of the secondary air passage having thecomplicated structure, whereas the portion for bonding and fixing thecircuit board, the housing, and other components is desired to be thin.

Under the circumstance, if such a thick part and such a thin part areformed as the same molded component, thermal contraction becomes unevenbetween the thick part and the thin part at the time of the molding, anda warpage phenomenon may occur at a boundary between the thick part andthe thin part. This warpage deformation causes a trouble in bondingproperties of the housing and the circuit board, and moreover,variations in a warpage amount lead to variations in a shape of a flowrate measuring unit. As a result, a new problem arises that the flowrate measuring accuracy also varies.

The present invention has been made in view of the above-mentionedpoints, and therefore has an object to provide a thermal type air flowmeter that is capable of suppressing deformation of a base member at thetime of molding, to thereby secure dimension accuracy and reduce aninfluence of a dimension change on measuring accuracy, thus enablinghighly accurate measurement of an air flow rate.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the present inventionprovides a thermal type air flow meter including: a housing memberplaced in an intake passage of an internal combustion engine; aplate-like base member that is fixed to the housing member and includesa secondary passage into which part of air passing through the intakepassage flows; a flow rate detecting element placed in the secondarypassage; and a circuit board that is electrically connected to the flowrate detecting element, receives an input of an amount of heat radiationfrom the flow rate detecting element, and outputs a signal according toan air flow rate. The base member is formed of a synthetic resinmaterial and includes: a board fixing part to which the circuit board isfixed; a secondary passage constituting part that is integrally formedso as to be continuous with a leading end part of the board fixing partand constitutes the secondary passage in cooperation with the housingmember; and a reinforcing structure that is provided at at least aconnection portion between the board fixing part and the secondarypassage constituting part and enhances strength of the base member.

With the thermal type air flow meter according to the present invention,the base member is formed of the synthetic resin material and includes:the board fixing part to which the circuit board is fixed; the secondarypassage constituting part that is integrally formed so as to becontinuous with the leading end part of the board fixing part andconstitutes the secondary passage in cooperation with the housingmember; and the reinforcing structure that is provided at at least theconnection portion between the board fixing part and the secondarypassage constituting part and enhances the strength of the base member.Accordingly, the strength of the flat plate-like base member is enhancedby the reinforcing structure.

As a result, at the time of molding the base member including: the boardfixing part formed of a thin part having a substantially constant resinthickness; and the secondary passage constituting part including a thickpart for forming the secondary passage, even in the case where thermalcontraction is uneven between the thin part and the thick part, warpagedeformation of the base member occurring at a boundary between the thickpart and the thin part can be suppressed.

Accordingly, it is possible to prevent a trouble of attachment to thehousing member and a trouble of attachment of the circuit board due tothe warpage deformation and also possible to prevent variations in theflow rate measuring accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a thermal type air flowmeter.

FIG. 2 is a development view illustrating components of FIG. 1.

FIGS. 3A and 3B are views each illustrating a conventional base member,FIG. 3A is a view illustrating the base member observed from its surfacejoined to a housing member, and FIG. 3B is a cross-sectional view ofFIG. 3A taken along the line A-A.

FIGS. 4A and 4B are views for describing a difference in cooling timebetween a thin part and a thick part, FIG. 4A is a cross-sectional viewcorresponding to FIG. 3B, and FIG. 4B is an enlarged view illustrating amain part of FIG. 4A.

FIGS. 5A and 5B are views each illustrating an example of warpagedeformation occurring in a base member, FIG. 5A is a cross-sectionalview corresponding to FIG. 3B, and FIG. 5B is an enlarged viewillustrating a main part of FIG. 5A.

FIGS. 6A, 6B, and 6C are views each illustrating an embodiment, FIG. 6Ais a front view illustrating the base member observed from its surfacejoined to the housing member, FIG. 6B is a side view observed from anupstream side in a forward flow direction, and FIG. 6C is a plan view.

FIGS. 7A, 7B, 7C, and 7D are views each illustrating another embodiment,FIG. 7A is a front view illustrating the base member observed from itssurface joined to the housing member, FIG. 7B is a side view observedfrom the upstream side in the forward flow direction, FIG. 7C is a planview, and FIG. 7D is a cross-sectional view of FIG. 7A taken along theline B-B.

FIGS. 8A, 8B, 8C, and 8D are views each illustrating still anotherembodiment, FIG. 8A is a front view illustrating the base memberobserved from its surface joined to the housing member, FIG. 8B is aside view observed from the upstream side in the forward flow direction,FIG. 8C is a plan view, and FIG. 8D is a cross-sectional view of FIG. 8Ataken along the line C-C.

FIGS. 9A, 9B, and 9C are views each illustrating still anotherembodiment, FIG. 9A is a front view illustrating the base memberobserved from its surface joined to the housing member, FIG. 9B is aside view observed from the upstream side in the forward flow direction,and FIG. 9C is a plan view.

FIGS. 10A, 10B, and 10C are views each illustrating still anotherembodiment, FIG. 10A is a front view illustrating the base memberobserved from its surface joined to the housing member, FIG. 10B is aside view observed from the upstream side in the forward flow direction,and FIG. 10C is a plan view.

FIG. 11 is a diagram illustrating an operation principle of the thermaltype air flow meter.

FIG. 12 is a view illustrating a control system of an internalcombustion engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described in detail withreference to the attached drawings.

First, an operation principle of a representative thermal type air flowmeter including a heat-generating resistor is described as an example ofan intake air measuring apparatus.

FIG. 11 is a schematic configuration circuit diagram illustrating theoperation principle of the thermal type air flow meter.

A drive circuit of the thermal type air flow meter is roughly formed ofa bridge circuit and a feedback circuit. The bridge circuit isconstructed by a heat-generating resistor RH for measuring an intake airflow rate, a temperature-sensitive resistor RC for compensating anintake air temperature, and resistors R10 and R11, and a heating currentIh is caused to flow through the heat-generating resistor RH such that aconstant temperature difference is maintained between theheat-generating resistor RH and the temperature-sensitive resistor RC,while performing feedback using an operational amplifier OP1, whereby anoutput signal V2 according to the air flow rate is outputted.Specifically, in the case where the flow rate is high, an amount of heattaken away from the heat-generating resistor RH is large, and hence alarger amount of the heating current Ih is caused to flow. In contrastto this, in the case where the flow rate is low, the amount of heattaken away from the heat-generating resistor RH is small, and hence theamount of the heating current Ih can be small.

Next, a configuration of a thermal type air flow meter according to thepresent invention is described with reference to FIG. 1.

In a primary air passage 250 constituted by an intake pipe constitutingmember 251, the left side of FIG. 1 corresponds to an air cleaner (notillustrated) side, and the right side thereof corresponds to an engine(not illustrated) side. With regard to an air flow direction, the airflow from the left side of FIG. 1 to the right side thereof is the airflow in the forward direction, is generally referred to as a forwardflow 252, and corresponds to a normal air flow when the air is takeninto the engine.

In contrast to this, the air flow from the right side of FIG. 1 to theleft side thereof is the air flow in the backward direction, and isgenerally referred to as a backward flow 253. The backward flow 253 isless likely to be generated in normal engine operation conditions. Inthe state where the intake air is pulsated by opening/closing of anintake/exhaust valve of the engine, if the rotation frequency of theengine and an n-th order wave having a vibration frequency specific toan intake pipe are synchronized with each other in a high-load statewhere a throttle valve (not illustrated) is opened to a large degree,the pulsation amplitude of the intake air pulsations becomes larger, andthe backward flow 253 is generated. Such a phenomenon is generallyknown.

In recent years, for exhaust gas regulations and reduced fuelconsumption, there is a demand to measure with high accuracy the intakeair flow rate. The thermal type air flow meter is strongly required toenable highly accurate measurement of the intake air flow rate in whichthe air flow rate of even the backward flow 253 is measured.

Similarly, a thermal type air flow meter 200 illustrated in FIG. 1includes a flow rate detecting element 201 having a function of not onlymeasuring the intake air flow rate but also detecting the flowdirection, to thereby measure the air flow rate of the forward flow 252and the air flow rate of the backward flow 253 in distinction from eachother. It should be noted that a known technique is used for a method ofmeasuring the forward flow 252 and the backward flow 253, and hencedescription of the method is omitted in the present invention.

The flow rate detecting element 201 is provided inside of a secondaryair passage (secondary passage) 202. A passage entrance 203 of thesecondary air passage 202 is opened so as to be opposed to the forwardflow direction, a passage exit 204 thereof is opened so as to be opposedto the backward flow direction, and the secondary air passage 202 has apassage structure that facilitates introduction of each air flow intothe secondary air passage 202. The flow rate detecting element 201 ismechanically fixed and electrically connected onto a circuit board 205constituting a drive circuit of the thermal type air flow meter 200,similarly to an electronic circuit component 206 such as an IC chip.

The circuit board 205 is electrically connected to a connector terminal208 via an aluminum wire 207, receives inputs from a power supply and aground, and outputs an air flow rate signal to the outside. The thermaltype air flow meter 200 is mechanically connected to the intake pipeconstituting member 251 by a fixing member such as a screw 209.

FIG. 2 is a component development view illustrating a componentstructure of the thermal type air flow meter illustrated in FIG. 1.

The thermal type air flow meter 200 includes, as main components, fourcomponents of a housing member 211, a cover member 220, the circuitboard 205, and a base member 230.

The housing member 211 is formed of a molded component of a syntheticresin such as plastic or a metal cast component, and is constituted by asubstantially rectangular plate-like member having a predetermined platethickness. The housing member 211 extends in a direction orthogonal tothe air flow inside of the intake pipe constituting member 251, and isattached such that both flat surfaces thereof are positioned along theair flow. A flange part for fixing the intake pipe constituting member251 and the connector terminal 208 for electrically connecting thecircuit board 205 with an external device are provided in a base endpart of the housing member 211.

The housing member 211 includes: a base member attachment surface on oneside, on which the base member 230 is attached; and a cover memberattachment surface on another side, on which the cover member 220 isattached. An opening hole 212 is formed in the base end part of thehousing member 211, and the electronic circuit component 206 on thecircuit board 205 fixed to the base member 230 can be inserted into theopening hole 212. A housing groove part 213 is provided on the covermember attachment surface in the leading end part of the housing member211, and the housing groove part 213 forms a second passage part 202B ofthe secondary air passage 202 in cooperation with the cover member 220.

The housing groove part 213 is formed so as to extend in the forwardflow direction from one side in the shorter-side direction located onthe upstream side in the forward flow direction toward another side inthe shorter-side direction located on the downstream side in the forwardflow direction. An upstream-side end of the housing groove part 213 isopened so as to pass through to the base member attachment surface side,and a downstream-side end thereof is formed so as to be continuous up toan end part on the another side in the shorter-side direction. Then, thecover member 220 is joined and combined with the cover member attachmentsurface of the housing member 211, to thereby cover an opened portion ofthe housing groove part 213, so that the second passage part 202B thatis part of the secondary air passage 202 and the passage exit 204 areconfigured.

The cover member 220 is formed of a plate-like member by molding a resinor other materials and has a size large enough to cover the openedportion of the housing groove part 213.

The circuit board 205 is electrically connected to the flow ratedetecting element 201, receives an input of the amount of heat radiationfrom the flow rate detecting element 201, and outputs a signal accordingto the air flow rate. Various electronic circuit components 206 andconductor wiring (no reference symbol) are placed on the circuit board205, the flow rate detecting element 201 is also mechanically andelectrically connected to the circuit board 205, and the flow ratedetecting element 201 is exposed in the secondary air passage 202.

The base member 230 is a plate-like resin molded component made of asynthetic resin material and includes: a flat plate-like board fixingpart 301 to which the circuit board 205 is bonded and fixed; and asecondary passage constituting part 302 that forms a first passage part202A that is part of the secondary air passage 202.

The board fixing part 301 has a size appropriate to close the openinghole 212 of the housing member 211 when the board fixing part 301 isattached to the base member attachment surface of the housing member211, whereby the circuit board 205 is sandwiched and housed between theboard fixing part 301 and the housing member 211.

A base groove part 214 is provided in the secondary passage constitutingpart 302, and the base groove part 214 forms the first passage part 202Athat is part of the secondary air passage 202, in cooperation with thehousing member 211 when the base member 230 is joined and combined withthe base member attachment surface of the housing member 211.

The base groove part 214 includes: a passage portion 214 a that isprovided so as to extend from the upstream side toward the downstreamside in the forward flow direction; a passage portion 214 b bent towardthe board fixing part 301 at an end part of the passage portion 214 a;and a passage portion 214 c that is bent at an end part of the passageportion 214 b and extends from the downstream side toward the upstreamside in the forward flow direction along the passage portion 214 a. Thebase groove part 214 is formed such that an end part of the passageportion 214 c is positioned so as to be opposed to and communicated withthe upstream-side end of the housing groove part 213 that is opened soas to pass through the housing member 211.

FIG. 3A to FIG. 5B are views for describing a factor that causes warpagedeformation of the base member. FIGS. 3A and 3B are views eachillustrating only a state where the circuit board 205 is mounted on thebase member 230, FIG. 3A is a view illustrating the base member 230observed from its surface joined to the housing member 211, and FIG. 3Bis a cross-sectional view of FIG. 3A taken along the line A-A. FIGS. 4Aand 4B are views for describing a difference in cooling time between athin part and a thick part, and FIGS. 5A and 5B are views eachillustrating an example of the warpage deformation occurring in the basemember. FIG. 4A and FIG. 5A are cross-sectional views each correspondingto FIG. 3B, and FIG. 4B and FIG. 5B are enlarged views illustrating amain part of FIG. 4A and a main part of FIG. 5A, respectively.

The base member 230 is a resin component integrally including the boardfixing part 301 and the secondary passage constituting part 302 and isformed by, for example, injection molding. In the case where the thermaltype air flow meter 200 is used for an automobile engine, because theusage environment is harsh, it is general to form the base member 230 byinjection molding of polybutylene terephthalate (PBT) resin or otherresins called engineering plastic.

The injection molding is a processing method in which a material of theresin is melt at a temperature of 250° C. or higher, is injected byapplying a pressure so as to fill a molding die, and thus is molded.After the molding, the material of the resin is left until thetemperature thereof cools down, and then is taken out as a completedresin component.

In general, if a resin thickness is made larger in a flat portion 401that extends in a flat plate-like pattern at a constant plate thickness,such as the board fixing part 301, slight irregularities called a sinkoccur after the molding. Accordingly, the flat portion 401 as describedabove is generally molded so as to have a thin resin thickness.

In contrast to this, in the secondary passage constituting part 302, itis necessary to form not only the flat portion 401 but also a verticalwall 402 that erects vertically from the flat portion 401, and hence theshape of the secondary passage constituting part 302 is complicated.Accordingly, a resin thickness of the secondary passage constitutingpart 302 is not constant, and the resin thickness (t2) thereof is largerthan the thin resin thickness (t1) of the flat portion 401 (t2>t1). Thatis, as illustrated in FIG. 4B, a thin part 411 and a thick part 412having different resin thicknesses exist in one resin component (basemember).

In FIG. 4B, for ease of description, a wall portion vertical (verticalwall 402) to the bonding surface of the circuit board 205 is illustratedas the thick part 412, but this wall portion does not necessarilycorrespond to the thick part 412.

As described above, in the case of the injection molding, the resin thathas been melt at a high temperature is injected into the molding die andthus is molded, and hence if a portion having a different resinthickness exists, there occurs a difference in time until the resintemperature cools down after the molding. That is, as illustrated inFIG. 4B, even if the resin temperature has cooled down in the thin part411, a phenomenon that the resin temperature has cooled down on thesurface of the resin but has not completely cooled down inside thereofoccurs in the thick part 412, so that a portion 413 in which cool-downis delayed is generated.

If there occurs a difference in time until the resin temperature coolsdown between the thin part 411 and the thick part 412 in this way, evenwhen the resin temperature has completely cooled down, a residual stressremains inside of the resin. If such a residual stress remains,deformation called warpage occurs in the shape of the base member 230 asillustrated in, for example, FIG. 5A.

For example, in the structure as illustrated in FIG. 5A, the left sideof the secondary passage constituting part 302 of the base member 230 inFIG. 5A is in a restricted state 501 in which a movement is structurallyrestricted by a resin wall, whereas the right side of the secondarypassage constituting part 302 in FIG. 5A is in an opened state 502 inwhich the structure is opened by the base groove part 214 constitutingthe secondary air passage 202.

For this reason, due to an influence of the residual stress, the basemember 230 is fixed on the restricted side, and the residual stress isreleased in the direction in which the base member 230 is opened.Accordingly, the base member 230 has a shape opened on thestress-released side, and as a result, has a shape inclined toward therestricted side (the left side of FIG. 5A) as illustrated in FIG. 5A, sothat deformation called warpage occurs.

If such warpage deformation occurs and the warpage amount varies foreach base member 230, variations occur in a flow passage area of thesecondary air passage 202 and in a distance between an inner wall of thesecondary air passage 202 and the flow rate detecting element 201, andthese variations may affect the measuring accuracy of the flow rate. Inaddition, the warpage deformation of the base member 230 may affect thejoining to the housing member 211 and the bonding property of thecircuit board 205. Accordingly, it is necessary to minimize as far aspossible the warpage deformation of the base member 230 at the time ofthe molding.

FIGS. 6A, 6B, and 6C are views each illustrating an embodiment of thebase member, FIG. 6A is a front view illustrating the base memberobserved from its surface joined to the housing member, FIG. 6B is aside view observed from the upstream side in the forward flow direction,and FIG. 6C is a plan view.

The base member 230 is provided with a rib 231 as a reinforcingstructure for enhancing the strength of the base member 230. Accordingto the same molding method as that of the base member 230, the rib 231is molded at the same time as the base member 230 with the use of thesame resin material and the molding die.

The circuit board 205 (illustrated by a dotted line) on which the flowrate detecting element 201 is placed and the base member 230 are bondedand fixed to each other (an adhesive is not illustrated), and the rib231 is provided such that a rear surface thereof faces the bondingsurface of the circuit board 205. Specifically, the rib 231 is providedso as to protrude from an outer surface of the board fixing part 301 ona side thereof opposite to the housing member 211, and the rib 231 isset at a connection portion between the thin board fixing part 301 towhich the circuit board 205 is bonded and the thick secondary passageconstituting part 302 in which the wall surface of the first passagepart 202A is formed, so as to join together the two parts.

In the present embodiment, the rib 231 is formed so as to protrude fromthe outer surface of the board fixing part 301 at a central position inthe shorter-side direction of the base member 230 and extend from thebase end part to the leading end part of the board fixing part 301.Accordingly, for example, in the case where the residual stress thatcauses the warpage as illustrated in FIG. 5A is applied, the rib 231serves to prop the structure, whereby the warpage of the base member 230can be suppressed.

FIGS. 7A, 7B, 7C, and 7D are views each illustrating another embodimentof the base member, FIG. 7A is a front view illustrating the base memberobserved from its surface joined to the housing member, FIG. 7B is aside view observed from the upstream side in the forward flow direction,FIG. 7C is a plan view, and FIG. 7D is a cross-sectional view of FIG. 7Ataken along the line B-B.

A characteristic feature of the present embodiment is that a rib 232 isprovided as a reinforcing structure, and the rib 232 extends from theboard fixing part 301 to the vicinity of a flow rate measuring part ofthe secondary passage constituting part 302. It should be noted thatconstituent elements similar to those in the above-mentioned embodimentare denoted by the same reference symbols, to thereby omit detaileddescription thereof. In addition, in FIGS. 7A to 7D, illustration of thecircuit board 205 and the flow rate detecting element 201 is omitted.

In the case where the flow rate detecting element 201 of the thermaltype air flow meter 200 is placed in the secondary air passage 202, itis necessary to measure a stable flow of a fluid to be measured. In ageneral method adopted for this purpose, the narrowest portion in whicha cross-sectional area inside of the secondary air passage 202 is thenarrowest is used as a flow rate measuring part, and the flow ratedetecting element 201 is placed in the flow rate measuring part. Anexample of the generally-adopted method includes providing anarrowed-down part 215 inside of the secondary air passage 202 tothereby temporarily narrow down the flow.

The embodiment in which the narrowed-down part 215 is provided inside ofthe secondary air passage 202 is illustrated in FIGS. 7A to 7D. Asillustrated in FIG. 7D, the narrowed-down part 215 has a convex shapethat protrudes inside of the secondary air passage 202 at asubstantially central position of the passage portion 214 c of the basegroove part 214, and narrows the flow passage area of the secondary airpassage 202 in the width direction.

Accordingly, an outer wall portion of the secondary passage constitutingpart 302 corresponding to a rear surface of the narrowed-down part 215has a concave shape. The present embodiment adopts a structure in whichthe rib 232 extends up to the outer wall portion. With this structure,one continuous rib 232 passes from the board fixing part 301 to thevicinity of the flow rate measuring part. Accordingly, it is possible toachieve stabilization of the fluid to be measured inside of thesecondary air passage 202 as well as further enhancement in strength ofthe base member 230.

With the above-mentioned structure, because the rib 232 extends up tothe vicinity of the flow rate measuring part, variations in the flowrate measuring accuracy can be suppressed. Dimension changes such as thecross-sectional area of the flow rate measuring part inside of thesecondary air passage 202 and the distance between the flow passage walland the flow rate detecting element 201 quickly affect the flow ratemeasuring accuracy. It is preferable that the flow rate detectingelement 201 measure the air flow in a stable state, and hence the flowrate detecting element 201 is set at a position having the narrowestcross-sectional area inside of the secondary air passage 202. Thecross-sectional area near the passage entrance 203 and the passage exit204 of the secondary air passage 202 is larger than that of the flowrate measuring part, and hence even if a dimension variation of ±0.1 mmexists, a change in the passage cross-sectional area is small. Incontrast, even with the same dimension change of ±0.1 mm, a change inthe cross-sectional area of the flow rate measuring part in which thepassage cross-sectional area is the narrowest is relatively largecompared with those of the passage entrance 203 and the like of thesecondary air passage 202, so that a flow rate measuring erroraccordingly becomes larger. As a matter of course, if the vector of theflow in the flow rate measuring part is changed by an influence ofwarpage, this directly leads to the flow rate measuring error, and hencethe flow rate measuring accuracy can be enhanced by reducing such adimension change near the flow rate detecting element 201. In thethermal type air flow meter 200 of the present invention, because therib 232 extends up to the vicinity of the flow rate measuring partinside of the secondary air passage 202, the rigidity of the flow ratemeasuring part is high, and a change in shape and dimensions of the flowrate measuring part can be reduced, so that the variations in the flowrate measuring accuracy can be suppressed.

FIGS. 8A, 8B, 8C, and 8D are views each illustrating still anotherembodiment of the base member, FIG. 8A is a front view illustrating thebase member observed from its surface joined to the housing member, FIG.8B is a side view observed from the upstream side in the forward flowdirection, FIG. 8C is a plan view, and FIG. 8D is a cross-sectional viewof FIG. 8A taken along the line C-C.

A characteristic feature of the present embodiment is that a rib 233 isprovided with a hollowed-out part 234 for making the resin thickness ofthe base member 230 even, and the hollowed-out parts 234 are provided ata plurality of points. In FIGS. 8A to 8D, the hollowed-out parts 234 areprovided at totally six points, that is, five points in the board fixingpart 301 and one point in the secondary passage constituting part 302,but the number of the hollowed-out parts 234 is not particularlylimited.

Similarly to the rib 232 of FIG. 7, the rib 233 is formed so as toextend up to the rear surface of the narrowed-down part 215. Because thebase member 230 is provided with the hollowed-out part 234, a thick partformed by the rib 233 is removed, the resin thickness of the entire basemember 230 is made even, a difference in time until the resin cools downfrom high temperature is eliminated, and the dimension accuracy can besecured.

Then, the hollowed-out part 234 thus provided to the rib 233 furtheradvantageously serves to enhance the dimension accuracy of the basemember 230.

The rib 233 provided to the base member 230 serves to enhance thestrength of the base member 230, but partially forms a thick part on thebase member 230. The thick part causes a difference in cooling timeuntil the resin cools down from high temperature after the molding,between the thick part and the thin part, and distortion may occur inthe base member 230.

Similarly to warpage, such distortion is a factor for deteriorating thedimension accuracy of the base member 230. Accordingly, in the presentembodiment, because the rib 233 is provided with the hollowed-out part234, the thick part is removed from the base member 230, a difference intime until the resin cools down from high temperature is eliminated, andthe dimension accuracy can be secured.

FIGS. 9A, 9B, and 9C are views each illustrating still anotherembodiment of the base member, FIG. 9A is a front view illustrating thebase member observed from its surface joined to the housing member, FIG.9B is a side view observed from the upstream side in the forward flowdirection, and FIG. 9C is a plan view.

A characteristic feature of the present embodiment is that the ribs 233and the hollowed-out parts 234 as illustrated in FIG. 8 are providedover an entire outer surface of the board fixing part 301. In thestructure described in the present embodiment, the plurality of ribs 233and the plurality of hollowed-out parts 234 are provided only on theouter surface of the board fixing part 301, but the ribs 233 and thehollowed-out parts 234 may be placed similarly on an outer surface ofthe secondary passage constituting part.

Particularly in the present embodiment, the ribs 233 are provided in agrid pattern, and hence the dimension accuracy of the base member 230can be further enhanced. As described above in the another embodiment,for example, even the same dimension change of ±0.1 mm has a largeinfluence on the flow rate measuring part and has a small influence onthe passage entrance 203 and the passage exit 204 of the secondary airpassage 202.

However, if a dimension change is small also at the passage entrance 203and the passage exit 204 on which the influence is small, the flow ratemeasuring accuracy is further enhanced. Accordingly, as illustrated inFIGS. 9A to 9C, the ribs 233 are provided in the grid pattern, and thedimension accuracy of the entire base member 230 is enhanced, wherebythe flow rate measuring accuracy of the thermal type air flow meter 200can be further enhanced.

FIGS. 10A, 10B, and 10C are views each illustrating still anotherembodiment of the base member 230, FIG. 10A is a front view illustratingthe base member observed from its surface joined to the housing member,FIG. 10B is a side view observed from the upstream side in the forwardflow direction, and FIG. 10C is a plan view.

A characteristic feature of the present embodiment is that a rib 235serving as a reinforcing structure is partially provided at theconnection portion between the board fixing part 301 and the secondarypassage constituting part 302 of the base member 230. The rib 235 isprovided so as to be interposed between the outer surface of the boardfixing part 301 and a vertical surface of the secondary passageconstituting part 302 that erects at an end part of the board fixingpart 301, and the rib 235 has a right triangle shape that is inclined soas to gradually approach the secondary passage constituting part 302toward a side farther from the outer surface of the board fixing part301. Then, a plurality of the ribs 235 are provided at a predeterminedinterval in the shorter-side direction of the base member 230corresponding to the forward flow direction, and in the presentembodiment, the ribs 235 are provided at totally three points.

Even with the rib 235 having such a simple structure, a sufficienteffect of suppressing warpage deformation of the base member 230 can beobtained. In the present embodiment, the number of the ribs 235 is threebut is not limited thereto as long as the effect can be obtained, andhence the number thereof may be increased or decreased.

FIG. 12 is a view illustrating an embodiment in which the presentinvention is applied to an electronic fuel injection type internalcombustion engine. Intake air 67 taken in from an air cleaner 54 passesthrough the intake pipe constituting member 251 of the thermal type airflow meter 200, an intake duct 55, a throttle body 58, and an intakemanifold 59 including an injector 60 to which fuel is supplied, andthen, the intake air 67 is taken into an engine cylinder 62. On theother hand, exhaust gas 63 generated in the engine cylinder 62 isdischarged through an exhaust manifold 64.

A control unit 66 receives inputs of: an air flow rate signal and apressure signal outputted from the circuit board 205 of the thermal typeair flow meter 200; an intake air temperature signal outputted from atemperature sensor; a throttle valve angle signal outputted from athrottle angle sensor 57; an oxygen concentration signal outputted froman oxygen concentration meter 65 provided to the exhaust manifold 64; anengine rotation speed signal outputted from a rotation speed meter 61;and other signals. The control unit 66 performs sequential arithmeticprocessing on these signals, to thereby obtain an optimal fuel injectionamount and an optimal opening degree of an idle air control valve, andcontrols the injector 60 and the idle air control valve by using theobtained values.

With the thermal type air flow meter 200 according to the presentinvention, the plate-like base member 230 is formed by molding asynthetic resin material, and one or more ribs 231 that are integrallyformed by using the same resin material as that of the base member 230are provided on the outer surface of the board fixing part 301 to whichthe circuit board 205 is bonded and fixed, so that the strength of thebase member 230 is enhanced. Accordingly, even in the case where boththe thin part 411 and the thick part 412 exist in the base member 230formed of a single component and thermal contraction is uneventherebetween at the time of the molding, it is possible to suppresswarpage deformation of the base member 230 occurring at a boundarybetween the thick part 412 and the thin part 411.

The present invention is not limited to the structures described abovein the embodiments and thus can be variously changed within a range notdeparting from the gist of the present invention. For example, anelement capable of measuring an intake air flow rate and also measuringan air flow direction at the same time may be used as the flow ratedetecting element 201. In the first place, a thermal type air flow meterhaving a function of only measuring a flow rate in a single directiondoes not need to have the complicated secondary air passage structure,and for example, even the secondary air passage structure as describedin JP Patent No. 3523022 fulfills a function as the secondary airpassage. In the case of the thermal type air flow meter that measuresthe intake air flow rate and also measures the air flow direction at thesame time, effects of the present invention are more advantageous.

At present, environmental issues such as global warming are attractingattention on a global scale. Accurate measurement of an intake air flowrate leads to optimal fuel control of a vehicle, and this enables notonly purification of exhaust gas but also realization of reduced fuelconsumption as countermeasures against the environmental issues typifiedby the global warming. As a result, it is possible to utilize to themaximum fossil fuels whose reserves are considered to be limited. Withan internal combustion engine to which the thermal type air flow meteraccording to the present invention is applied, a flow rate can beobtained with high measuring accuracy, and optimal fuel control can beperformed.

DESCRIPTION OF SYMBOLS

51 . . . intake air temperature sensor, 54 . . . air cleaner, 55 . . .duct, 56 . . . idle air control valve, 57 . . . throttle angle sensor,58 . . . throttle body, 59 . . . intake manifold, 60 . . . injector, 61. . . rotation speed meter, 62 . . . engine cylinder, 63 . . . gas, 64 .. . exhaust manifold, 65 . . . oxygen concentration meter, 66 . . .control unit, 67 . . . intake air, 200 . . . thermal type air flowmeter, 201 . . . flow rate detecting element, 202 . . . secondary airpassage, 203 . . . passage entrance, 204 . . . passage exit, 205 . . .circuit board, 206 . . . electronic circuit component, 207 . . .aluminum wire, 208 . . . connector terminal, 209 . . . fixing screw, 210. . . air flow inside of secondary air passage, 211 . . . housingmember, 220 . . . cover member, 230 . . . resin base member, 231 . . .rib, 233 . . . rib, 234 . . . hollowed-out part, 235 . . . rib, 250 . .. primary air passage, 251 . . . intake pipe constituting member, 252 .. . forward flow inside of primary air passage, 253 . . . backward flowinside of primary air passage.

What is claimed is:
 1. A thermal type air flow meter for being placed inan intake passage of an internal combustion engine, the thermal type ofair flow meter comprising: a resin component having a secondary passageinto which part of air passing through the intake passage can flow; anda circuit board configured to output a signal according to an air flowrate based on a signal from a flow rate detecting element disposed onthe circuit board, the circuit board on which the flow rate detectingelement is disposed being installed so that the flow rate detectingelement is placed in the secondary passage, wherein the resin componentcomprises: a board fixing part to which the circuit board is fixed, asecondary passage constituting part that is a part of the secondarypassage, and ribs provided on a rear surface which is a side opposite toa surface of the board fixing part, wherein the ribs are provided on therear surface in a grid pattern.
 2. The thermal type air flow meteraccording to claim 1, wherein the widths of adjacent ribs are equal. 3.The thermal type air flow meter according to claim 2, wherein aplurality of the hollowed-out parts are square.
 4. The thermal type airflow meter according to claim 2, wherein the resin component is molded.5. The thermal type air flow meter according to claim 1, wherein theribs comprise a plurality of hollowed-out parts.
 6. The thermal type airflow meter according to claim 5, wherein the plurality of thehollowed-out parts are provided at least along a longer side directionof the thermal air flow meter.
 7. The thermal type air flow meteraccording to claim 6, wherein the secondary passage constituting partincludes a narrowed-down portion of the secondary passage, and one ofthe hollowed-out parts is located in the secondary passage constitutingpart opposite to the narrowed-down portion.
 8. The thermal type air flowmeter according to claim 6, wherein the ribs extend from a base end partof the board fixing part which is an end part on a flange side of theboard fixing part.